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University of Toronto St. George

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Human Biology

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HMB300H1

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j

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Winter

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Chapter 14- Chemical Kinetics
Nerosanth Selvarajah
14.1- The Rate of a Chemical Reaction
• The rate of a reaction describes how fast the concentration of a reactant or product changes with time
A + B  C + D
Rate of Formation = ∆[C]/ ∆t Rate of Disappearance = -∆[A]/ ∆t
• Rate of disappearance is a negative quantity because concentration decreases with time
o The concentration at the end of a time period is less than t was at the start of the period
14.2 Measuring Reaction Rates
• To determine rate of reaction, we need to measure changes in concentration over time
• Reaction rate is not constant; the lower the remaining concentration of the reactant, the more slowly the
reaction proceeds
• Instantaneous Rate of Reaction- is the exact rate of a reaction at some precise point in the reaction. It is
obtained from the slope of a tangent line to a concentration-time graph
• Initial Rate of Reaction- is the rate of a reaction immediately after the reactants are brought together
14.3 Effect of Concentration on Reaction Rates: The Rate Law
• Rate Law/ Rate Equation-for a reaction relates the reaction rate to the concentrations of the
reactants
m n
Rate= k[A] [B] [Pg. 578]
• Term order is related to the exponents in the rate law
• The overall order of reaction is the sum of all the exponents: m + n + …
• Rate constant (k)- is the proportionality constant in a rate law that permits the rate of a reaction to be
related to the concentrations of the reactants
o its values depend on the specific reaction, presence of a catalyst and temperature
o the larger the value of k, the faster a reaction goes
• order of the reaction establishes the general form of the rate law & the appropriate units of k
• if reaction is first order in one of the reactants, doubling the initial concentration of that reactant causes the
initial rate of reaction to double
o zero order in reactant- no effect on initial rate of reaction
o first order in reactant- initial rate of reaction doubles
o second order in reactant- initial rate of reaction quadruples
o third order in reactant- initial rate of reaction increases eightfold
• order of reaction (indicated through rate law) establishes units of rate constant, k [k = M min -1
14.4 Zero-Order Reactions
• zero-order reaction has a rate law in which the sum of the exponents, m + n+… is equal to 0
o reaction proceeds at a rate that is independent of reactant concentrations
Rate of Reaction = k [A] = k = constant
 concentration-time graph is a straight line with a negative slope
 rate of reaction which is equal to k and remains constant throughout the reaction is the
negative of the slope of the line
 units of k are the same as units of rate of a reaction: mol/L*t = M/s
• Integrated Rate Law- expresses the concentration of a reactant as a function of time
[A]t= -kt + [A]o
14.5 First-Order Reactions
 First-order reaction has a rate law in which sum of the exponents, m + n + …is equal to 1
ln ([A]t/ [A]o) = -kt or ln [A]t= -kt + ln[A] o
 an easy test for a first-order reaction is to plot the natural logarithm of a reactant concentration versus time
& see if graph is linear k = -slope
 Half-Life of a reaction is the time required for one-half of a reactant to be consumed; time during which the
amount of reactant or its concentration decreases to one-half of its initial value
t1/2= ln 2 / k
 Half-life is constant for a first-order reaction; it is also independent of the initial concentration
used
 In Reactions involving gases, Rates are often measured in terms of gas pressure
 Radioactive decay is a first-order process
14.6 Second-Order Reactions Equation of Straight Line Graph:
1/ [A]t= kt + 1/[A] 0
 Half life depends on both the rate constant & the initial concentration [A]
0
o Half –life is not a constant; its value depends on the concentration of reactant at the start of each
half-life interval
 Because the starting concentration is always one-half that of the previous half-life, each
successive half-life is twice as long as the one before it
t1/2= 1/ k[A] 0
 Pseudo-First-Order Reaction- a second-order reaction that is made to behave like a first-order reaction by
holding one reactant concentration constant
14.7 Reaction Kinetics: A Summary
REFER TO PAGE 589
14.8 Theoretical Models for Chemical Kinetics
Collision Theory:
 Colli30on Theory- the number of molecular collisions per unit time; typical collision frequency is of the order
of 10 collisions/second
 Only a fraction of the collisions among gaseous molecules lead to chemical reaction
 Cannot expect every collision to result in a reaction
 Activation Energy- a reactions minimum energy above the average kinetic energy that molecules must
bring to their collisions for a chemical reaction to occur
 Rate of a reaction wi